The present invention relates to an optical disk apparatus for recording/reproducing information by using a light beam and, more particularly, to an optical disk apparatus for recording information on grooves and/or lands formed in an optical recording medium.
An optical disk apparatus records information on and reproduces recorded information from grooves and/or lands formed in an optical recording medium by utilizing optical magnetism or phase change. Generally, a laser beam must irradiate the grooves and lands after it is focused by using an objective lens. For this purpose, a focal position shift of the laser beam is detected, and focusing of the objective lens is controlled on the basis of the detected focal position shift. Conventionally, a method called astigmatic method is widely used as a method of detecting this focal position shift.
According to this method, as shown in FIG. 4, an exit beam from a laser source 1 is focused by an objective lens 4 to form a fine spot through a beam splitter 3, to irradiate the recording surface of an optical disk 5. The beam reflected by the optical disk 5 is split by the beam splitter 3 and converted by an astigmatic element 7, e.g., a cylindrical lens, that generates astigmatism into an astigmatic beam in which its focal positions differ between the X and Y directions on a plane perpendicular to the traveling direction of the beam, as shown in FIG. 5. The astigmatic beam emerging from the astigmatic element 7 is received by a 4-division photodetector 8 constituted by 4 photodetection elements to detect its focal shift amount.
More specifically, when the 4-division photodetector 8 is set at an intermediate position A, B, or C (FIG. 5) of a distance L between the two focal points of the astigmatic beam in the optical axis direction such that the dividing directions of the 4 detection elements form an angle of 450 with respect to the X and Y directions of the astigmatic beam, the beam shape on the 4-division photodetector 8 changes in accordance with the focal shift amount (positions A to C) as shown in FIGS. 6A to 6C.
When detection signals detected by divisional regions (photodetection elements) 8a and 8d, and 8b and 8c at the diagonal positions of the 4-division photodetector 8 are summed and the difference between the sum signals is calculated, a focal shift amount having an S-shaped curve as indicated by a solid line in FIG. 7 can be obtained. The zero-crossing point of this S-shaped curve is recognized as the in-focus position. An in-focus state on the optical disk can be obtained by adjusting the position of the objective lens with respect to the optical disk such that the focal shift amount becomes zero.
Generally, localization in diffracted beam distribution generated by a shift in the focusing spot position of the light beam, which is focused on a groove and land of an optical disk by an objective lens, occurs in a direction perpendicularly intersecting the direction of track length. According to the astigmatic method, when a difference between detection signals of two adjacent divisional regions of the 4-division photodetector 8 is obtained, the position shift signal of a focal position in the direction of track width, i.e., a tracking signal, can be detected simultaneously, and tracking adjustment can be performed simultaneously.
In the focal shift amount obtained by the astigmatic method described above, it is recognized that an offset occurs wherein the focal shift amount at the in-focus position does not become zero, as indicated by a broken line in FIG. 7, particularly when grooves and lands are formed in the recording surface of the optical disk. When information is recorded on both the grooves and lands as in an optical disk in recent high-density recording, if the focal shift amount is to be detected by the astigmatic method, the offset becomes different between the grooves and lands, as indicated by a broken line in FIGS. 8A and 8B.
According to the measurement done by the present inventor, the difference in focal shift amount between the grooves and lands is considerably larger than the depth (about 80 nm with a wavelength of 650 nm) 1/8 a wavelength .lambda. of the laser source, which is the depth of the grooves employed in a general optical disk, and sometimes reaches a value of about 0.3 .mu.m. The depth of the grooves in this conventional case is the one with which the track error signal in the push-pull method becomes the maximum when tracking adjustment is performed by utilizing a diffracted beam, as described above.
When such a large focal shift amount occurs, it cannot be moderated by only adjusting the position of the photodetector, and information recording on both the grooves and lands set at the optimum focal positions cannot be performed. This problem occurs not only when a finite-system objective lens is used, as shown in FIG. 4, but also when an infinite-system objective lens and collimator lens are used or a focusing lens is used independently in a reflected beam detection system.